This invention relates generally to optical focusing systems of the type utilizing a reduction lens system for photometrically printing an image of a first object such as a reticle upon a second object such as a semiconductive wafer and, more specifically, to apparatus utilized in such systems to achieve precise focusing or positioning of the top surface of the wafer at the image plane of a projection lens of the lens system.
In the semiconductor industry, reduction lens systems are employed in the fabrication of photomasks to print an array of microcircuitry on each photomask. A set of such photomasks, each bearing an array of microcircuitry of a different level, is typically employed in the fabrication of integrated circuits from a semiconductive wafer. In the course of this fabrication, the semiconductive wafer is sequentially aligned with each photomask, and an exposure is made at each processing level to print the array of microcircuitry on the photomask onto the semiconductive wafer. Alternatively, similar reduction lens systems may be directly employed in an exposure operation to initially print each level of microcircuitry directly upon the semiconductive wafer rather than a photomask, thereby eliminating the photomask printing step entirely. However, whether a reduction lens system is employed to print a level of microcircuitry initially on a photomask or directly on a semiconductive wafer, the top surface of either the photomask or the wafer must be precisely focused, automatically, at the image plane of a projection lens of the lens system. In practice, however, precise focusing of semiconductive wafers is very difficult to achieve because of surface irregularities present in the surface of the unprocessed (unexposed) wafer or introduced as a result of various processing steps, resulting in mottled exposure of photoresist and unresolved microcircuit images formed upon the wafer.
Known prior-art systems do not adequately provide for detection of focus error due, for example, to imprecise focusing of the image plane of the projection lens at the edges of the exposure (i.e. at the edges of the portion of the wafer being exposed, which edges are sometimes not in the same horizontal plane as the center of the wafer). One such prior-art system is described, for example, in U.S. Pat. No. 3,722,996 entitled OPTICAL PATTERN GENERATOR OR REPEATING PROJECTOR OR THE LIKE and issued Mar. 27, 1973, to Wayne L. Fox. Other prior-art systems also do not adequately provide for detection of focus error because they often utilize sample areas which are too small (e.g., extremely small sample areas of about ten microns in diameter are generally used). One such prior-art system is described, for example, in U.S. Pat. No. 4,128,847 entitled OPTICAL READING DEVICE COMPRISING A FOCUSING DETECTION DEVICE and issued Dec. 5, 1978, to Gerald Roullet and Jean-Pierre Bortuzzo.
What would be useful in providing improved microcircuit exposures, therefore, is a system that would be capable of detecting when a selected portion of a wafer, even a portion close to an edge of the wafer, is out of focus due to variations in thickness or flatness of the wafer or other factors, and capable of correcting the detected out-of-focus condition by bringing the selected portion into focus prior to exposure. Also, to increase the utility of such a system, and, hence, the production of microcircuits, it would be desirable that the system be ruggedly constructed and not affected by small variations in environmental factors, such as temperature, as are many prior-art systems.
Accordingly, it is the principal object of this invention to provide an optical focusing system capable of accurately focusing wafer surfaces despite non-parallel, and otherwise non-flat conditions of a wafer surface.
Another object of this invention is to provide an optical focusing system incorporating a full aperture focus detection apparatus capable of accurately detecting the focus condition of a wafer surface despite non-parallel and otherwise non-flat conditions of the wafer surface.
Another object of this invention is to provide an optical focusing system incorporating a full aperture focus detection apparatus capable of accurately detecting the focus condition of a wafer surface utilizing the existing projection lens included in the associated exposure system.
Another object of this invention is to provide an optical focusing system incorporating a full aperture focus detection apparatus capable of accurately detecting the focus condition of a wafer surface while the associated exposure system is operational.
Another object of this invention is to provide an optical focusing system incorporating a full aperture focus detection apparatus capable of accurately detecting the focus condition of a wafer surface despite differences in reflectivity of different portions of the wafer surface.
Another object of this invention is to provide an optical focusing system incorporating a lens positioning apparatus that may be ruggedly handled without damage to the system.
Another object of this invention is to provide an optical focusing system incorporating a kinematic lens positioning apparatus capable of restraining five degrees of freedom of movement and permitting precise, unrestrained movement in the sixth degree of freedom of movement.
Another object of this invention is to provide an optical focusing system incorporating a kinematic lens positioning apparatus permitting the system to be virtually perturbation free in the directions in which the system is constrained.
Another object of this invention is to provide an optical focusing system incorporating a kinematic wafer chuck differential leveling or positioning apparatus capable of leveling the chuck to compensate for non-parallel conditions of a wafer.
Still another object of this invention is to provide a kinematic system for positioning a lens along a single axis and a kinematic system for positioning a wafer chuck parallel to a selected plane.
These and other objects, which will become apparent from a reading of this specification and a review of the accompanying drawings, are accomplished according to the illustrated preferred embodiment of the present invention by providing an optical focusing system comprising a full aperture focus detection apparatus, a kinematic lens positioning apparatus, and a kinematic wafer chuck differential leveling or positioning apparatus. The full aperture focus detection apparatus detects the focus condition of a wafer relative to the image plane of a projection lens associated with the kinematic lens positioning apparatus, and actuates the kinematic lens positioning apparatus and/or the kinematic wafer chuck differential leveling or positioning apparatus to correct for an out-of-focus condition of the wafer and to accurately focus the surface of the wafer. In response to this actuation, the kinematic lens positioning apparatus makes coincident the image plane and the top surface of the wafer, and the kinematic wafer chuck differential leveling or positioning apparatus corrects for a non-parallel condition of the wafer surface by bringing discrete surface regions of interest into a common plane, namely the image plane of the projection lens.
The kinematic lens positioning apparatus includes a slide for supporting the projections lens, a drive mechanism for moving the slide, and constraining apparatus (including the drive mechanism) for kinematically constraining the slide and, hence, the projection lens to move along a single axis. The kinematic wafer chuck differential leveling or positioning apparatus includes first, second and third spaced drive mechanisms for supporting the wafer chuck for translational movement along a first axis parallel to the single axis of movement of the projection lens and for rotational movement about second and third axes orthogonal to the first axis. It also includes constraining apparatus (including the first, second and third drive mechanisms) for kinematically constraining the wafer chuck to move only along the first axis and about the second and third axes.